Stringed musical instrument with optoelectronic pickup sound amplifier

ABSTRACT

A stringed instrument, such as an acoustical guitar, is provided with a compact, battery powered sound amplification unit attached thereto in an arrangement including cooperating optoelectronic devices situated adjacent the instrument&#39;&#39;s strings. Each string intersects with a path of a light beam sent from a first lightemitting device toward a second light-detecting device so that vibration of a string modulates the intensity of light from the first device impinging on the second device to produce an electronic signal corresponding to the musical tone associated with any particular string vibration rate. The unit can be affixed to an acoustical guitar and utilized to drive a speaker or equivalent acoustic transducer situated within the sound box of the guitar so as to cause the transducer to produce amplified musical tones corresponding to the musical notes played on the strings of the guitar. The source of electric power for operating this device shall be situated on or within the guitar to provide a self-contained device that is not dependent on a source of domestic electric power.

Ullittfd States Patent 91 Ferber 1 3,733,953 [4 1 May 22, 1973 [54]STRINGED MUSICAL INSTRUMENT WITH OPTOELECTRONIC PICKUP SOUND AMPLIFIER[76] Inventor: Dennis A. Ferber, 11322 Donovan Road, Los Alamitos,Calif. 90720 [22] Filed: Dec. 30, 1971 [21] Appl. No.: 213,935

[52] U.S. Cl ..84/l.l6, 84/1.l8 [51] Int. Cl. ..Gl0h 3/00 [58} FieldofSearch ..84/l.14l.l6,

84/l.04,l.06, 1.18, DIG. 19; 317/235 N [56] References Cited UNITEDSTATES PATENTS 3,194,870 7/1965 Tondreau et a1. "84/1 16 3,514,5225/1970 Mussulman ..84/1.04 X 2,972,922 2/ 1961 Rhodes ..84/1.l53,038,363 6/1962 Miessner ..84/ l .04 X 3,514,522 5/1970 Musaulman ..84/1.14 X 2,972,922 2/ 1961 Rhodes ..'.84/1 .06 X 3,038,363 6/1962 Miessner..84/1.04 X 3,535,532 10/1970 Marryman ..317/235 N OTHER PUBLICATIONS F.W. Gutzwiller, G. 15., SCR Manual Page 289, 1967, G. E., ElectronicsPark, Syracuse 1, N.Y.

Primary Examiner-Richard B. Wilkinson Assistant ExaminerU. WeldonAttorney-William C. Babcock [5 7] ABSTRACT A stringed instrument, suchas an acoustical guitar, is provided with a compact, battery poweredsound amplification unit attached thereto in an arrangement includingcooperating optoelectronic devices situated adjacent the instrumentsstrings. Each string intersects with a path of a light beam sent from afirst lightemitting device toward a second light-detecting device sothat vibration of a string modulates the intensity of light from thefirst device impinging on the second device to produce an electronicsignal corresponding to the musical tone associated with any particularstring vibration rate.

The unit can be affixed to an acoustical guitar and utilized to drive aspeaker or equivalent acoustic transducer situated within the sound boxof the guitar so as to cause the transducer to produce amplified musicaltones corresponding to the musical notes played on the strings of theguitar. The source of electric .power for operating this device shall besituated on or within the guitar to provide a self-contained device thatis not dependent on a source of domestic electric power. 1

7 Claims, 8 Drawing Figures FDWEE FIELD AND BACKGROUND OF THE INVENTIONThe present invention relates to an optoelectronic unit utilizable withstringed musical instruments and responsive to string vibrations whenthe instrument is played in conventional fashion.

The present invention relates also to a sound amplifier system designedso that all of its component parts are situated within a portablestringed instrument such as an acoustical guitar.

The present invention in a detailed sense relates to a pickup deviceincluding cooperating optoelectronic devices, electrical amplifiercircuitry, and associated speaker in combination with the strings of amusical instrument in an arrangement wherein string vibrations aresensed (picked up) and transduced into corresponding electrical signalsthat are, in turn, transduced into corresponding audible tones afterhaving been electronically amplified.

Transducer pickups are utilized for transducing sounds produced inplaying stringed musical instruments into corresponding electronicsignals that can be electronically amplified and fed to a loudspeaker tocause it to produce audible sounds that can be heard from greatdistances. Modernly, pizeoelectric transducers have found popular usagewith such stringed instruments as the electric guitar and the bassfiddle. Pizeoelectric and other kinds of transducer pickups cannotaffordprecisely thesame results and advantages that are afforded by theoptoelectronic transducer pickup of the present invention, nor are theformer utilizable with all of the various kinds of stringed instrumentswith which the latter may be utilized.

The modern electric guitar utilizes a pizeoelectric transducerarrangement connected by electrical wires in a cord to a separateamplifier, power source and loudspeaker unit. Such a guitar differs froman acoustical guitar which, unlike the former, has a sound box definedin the body thereof for providing acoustical amplification, via a soundopening in the body, of the musical tones or sounds actually produced byplucking or strumming the guitar strings. In contrast to the foregoing,an actual embodiment of the present invention can be utilized inconjunction with an acoustical guitar, wherein the acoustic transducermakes advantageous use of the soundbox, with all of the componentsthereof optoelectronic transducer pickup arrangement, amplifier system,and power system contained in a detachable unit secured to theacoustical guitar forming a self-contained unit.

OBJECTS OF THE INVENTION It is accordingly an object of the presentinvention to provide a compact pickup and amplifier unit, small enoughto be affixedto a stringed musical instrument such as an acoustical orelectrical guitar It is another object of the present invention toprovide an optoelectronic transducer pickup system adapted forutilization with stringed musical instruments to produceoptoelectronically generated electronic signals corresponding infrequency to the frequency of the musical tones produced by effectingvibration of the instruments strings at various rates. It is anotherobject of the. present invention to provide a battery powered unit ofthe kind herein described that is designed to allow for the mechanicaladjustment of the relative positions of the instruments strings inrelation to its optoelectronic devices so that the unit can be utilizedin conjunction with different, but similar, stringed musical instrumentshaving different spacing between strings and other physical differences.

It is another object of the invention to provide an acoustical guitar orlike instrument with an acoustic transducer in the sound box thereof andwith a composite optoelectronic transducer, power and amplifier unitaffixed to the outside front face of the guitar body near the bridge ofthe guitar.

These and other further and related objects, advantages, and features ofthe present invention may be made apparent in light of the followingillustrative description of the present invention taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of asix-string acoustical guitar provided with an acoustic transducersituated inside the sound box of the guitar body and an optoelectronictransducer/amplifier/power unit removably attached to the outside frontface of the body over the guitar bridge at one end of the guitarstrings;

FIG. 2 is a view taken generally along line 2-2 of FIG. 1 illustratingan arrangement for photoelectrically sensing vibrations of any guitarstring so as to produce corresponding electronic signals that areamplified and then fed to the aforementioned loudspeaker;

FIG. 3 is a view taken along line 33 of FIG. 2 showing a single guitarstring in relation to a light source manifold and a photodetector andother elements;

FIG. 4 is a plan view taken along line 4-4 of FIG. 2 of a longitudinallyslotted bar-shaped member and two threaded bolts. This Figure, togetherwith FIGS. 2 and 3, is illustrative of one way in which theoptoelectronic transducers for each string may be arranged relativethereto.

FIGS. 5 and 6 are illustrative views taken orthogonal to each other,showing an acoustic transducer mounted inside the sound box of theguitar and located rearwardly of the guitars sound opening, with FIG. 5being taken along line 5-5 of FIG. 1;

FIG. 7 is an electrical schematic of circuitryfor photoelectricallysensing string vibrations so as to generate corresponding electronicsignals, amplifying such signals, and feeding the amplified signalsto anacoustic transducer; and

FIG. 8 is an electrical schematic of circuitry sensitive to light ofvariable intensity.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring initially to FIG. 1there is shown an acoustical guitar 10 which is representative ofvarious stringed musical instruments with which device arrangementsembodying the present invention may be utilized, and which has sixstrings. The guitar 10 is conventional and includes a sound opening 12in the front face of its main portion or body 14 which is hollow betweenits front and back faces so as to define therein a sound box. The guitar10, in accordance with the present invention, includes a unit 16 thatcontains all of the electrical components and circuitry depicted in FIG.7 with the exception of the sound reproduction device that may be eithera dynamic speaker or acoustic transducer which is preferably mountedwithin the sound box of the guitar body 14.

The unit 16, as illustrated, includes a casing 18 which, as will beexplained in greater detail hereinafter, encloses six sets ofoptoelectronic devices, amplifier circuitry, and a battery power supply.The casing 18 has four side walls forming an enclosure and is open atits frontmost side and its bottom side which is next to the front faceof the guitar body 14. There is a slidable switch button 20 extendingfrom the rearmost side wall of the unit which can be actuated whendesired to either turn on or turn off the supply of battery power to thecircuitry components disposed within the unit. Two opposite side wallsof the casing straddle the guitar bridge, to which one end of each ofthe six strings is secured. The casing 18 is secured to the body frontface in a suitable fashion, as by means of a machine screw 22 threadedinto a threaded hole in the top casing wall and extending downwardlybetween the two middle guitar strings in a threaded hole in the bodyfront face so as to secure the casing to the guitar body in such a waythat the casing can be detached readily from the guitar body whendesired.

Located within the casing 18 is a mechanical support assembly for sixsets of optoelectronic devices which can be seen in FIGS. 2 and 3. Thesupport assembly includes two bar-shaped frame members 24 and 26 held inspaced relation by machine screws extending therebetween. Attached tothe topmost frame member 24 and suspended therefrom are six separatesupport members 28, 30, 32, 34, 36, and 38 that are each generallyC-shaped when seen from the view shown in FIG. 2. Each of the supportmembers 28 to 38 are suspended from the frame member 24 in the same wayand in a manner that is shown at the left in FIG. 2, wherein a partialcross section is taken through the support member 28 in order to showhow it is suspended from the frame member 24. The support member 28, asare the other support members, is suspended in place by the utilizationof a first machine screw 39 with a slotted head 40 that passes through alongitudinal slot 42 (partially shown in FIG. 4) into a threaded hole 44in the support member 28. By loosening the screw 39 with a screwdriverit is possible to loosen the support member 28 from the frame member 24so that the support member 28 and frame member 24 no longer securelyabut one another, to allow the support member to be moved parallel tothe longitudinal direction of the frame member 24. In this way thesupport member 28 may be repositioned relative to string S1 whichextends between top and bottom ends of the support member 28 in adirection normal to an imaginary line drawn between the support memberends. This feature, just described, allows the support member to beaccurately positioned relative to the string S1 and also adapts thesupport member and frame member arrangement for utilization withdifferent guitars and other stringed instruments wherein the spacing ofthe'strings of the different instruments may vary to some extent. Theother support members 30, 32 34, 36, and 38 are suspended from the framemember 24 in the same way that support member 28 is suspended by theutilization of respective machine screws 46, 48, 50, 52, and 54.

The frame members 24 and 26 are held in spaced parallel relation bymeans of two frame machine screws 56 and 58 at opposite ends of theframe members that are each threaded into corresponding threaded holdsin the frame members 24 and 26. A first pair of threaded nuts 60 and 62and a second pair of threaded nuts 64 and 66 are utilized to determinethe spacing of frame members 24 and 26. The nuts 60 and 62 are threadedonto the frame machine screw 56 on opposite sides of the frame member 24and the nuts 64 and 66 are threaded onto the frame machine screw 58 onopposite sides of the frame member 24 so that by tightening the nutsagainst the frame member 24 it is possible to maintain the frame member24 at any desired spacing from frame member 26. FIG. 4 is a plan view ofthe left end of the frame member 24 showing, for further illustrativepurposes, the arrangement of the frame machine screw 56, threaded nut60, and the heads 40 and 68 of the machine screws 39 and 46 relative tothe frame member slot 42.

Referring further to FIG. 2, electrically operated light emitters D1,D2, D3, D4, D5, and D6 are secured to the top ends of the supportmembers 28 and 38 and light detecting sensors D7, D8, D9, D10, D11, andD12 are secured to the bottom ends of the support members 28 to 38 sothat the light emitter D1 faces the light sensor photodiode D7, theemitter D4 faces the sensor D10, the emitter D5 faces the sensor D11,and the emitter D6 faces the sensor D12. The light emitter and lightsensors D1 to D12 are optoelectronic devices that function in a mannerto be explained hereinafter. The emitters D1 to D6 are respectivelyaligned with the sensors D7 to D12 along respective axes that aretransverse to the guitar strings S1 and other guitar strings S2, S3, S4,S5, and S6 so that light rays emitted by each of the emitters D1 throughD6 form respective light beams that impinge respectively on the sensorsD7 through D12. The strings S1 through S6, being respectively locatedbetween the emitters D1 through D6 and the sensors D7 through D12,intersect with the respective light beams to control the amount oflight-energy from the emitters D1 through D6 reaching the respectivesensors D7 through D12 in accordance with string vibrations producedwhen the strings are plucked or strummed during the playing of theguitar.

When a musical note is played on any of the guitar strings, the stringvibrates at a rate or frequency depending upon the particular noteplayed. Each vibrating string moves sideways back and forth indirections generally normal to the light beam passing between theassociated emitter and sensor so as to chop the light beam at the samerate as the string vibration rate. The chopping of any light beamproduces a signal voltage in the particular photodiode illuminated bythe light beam, since the voltage produced in a sensor by impinginglight is proportional to the light energy contained in the light ray andsuch voltage signal is of a frequency corresponding to the vibrationrate or frequency of the associated string. The voltage signals producedin the foregoing manner are electronically amplified, as will beexplained, and fed to a sound reproducing device, earlier mentioned,mounted on or inside the guitar body, the device broadcasting themusical tones produced when musical notes are played on the guitarstrings.

The optoelectronic devices in pairs Dl-D7, D2-D8, D3-D9, D4-Dl0, DS-Dll,and D6-D12 respectively function as optoelectronic transducer pickups inthat each mentioned pair (e.g., D1-D7) serve to transduce the opticalenergy of a light beam into an electronic signal. These devices,together with the guitar strings S1 through S6 each constitute amechanoelectronic or mechano-optoelectronictransducer in that eacharrangement of diode, photodiode and string (e.g., D1, D7 and S1)transduce the mechanical motion of a guitar string into a correspondingelectronic signal.

Referring now to FIG. 3, there is shown an elevational view taken alongline 33 of FIG. 2 of the emitter D6 and sensor D12 on the support member38 as arranged relative to the string S6. The support member 38, asstated earlier, is suspended between the frame members 24 and 26 (whichare shown in cross section), the frame member 26 abutting the outsidefront face of the guitar body therebeneath. The string S6 extends to theright as shown in FIG. 3 into a vertical groove of a guitar bridge GB toan anchoring element A situated to the right of the guitar bridge.

A case C secured to the guitar bridge GB defines an enclosed space Voutline by dashed lines, into which the battery B1 can be inserted.Attached to the frame member 24 is a printed circuit board (PCB), withprinted circuit conductors formed on its top and bottom faces.Electrical lead wires interconnect the emitter D6 and the sensor D12 toelectrical conductors on the printed circuit board PCB. Disposed atopthe printed circuit board PCB are the circuit components of the circuitof FIG. 7, these components being represented in FIG. 3 by thecomponents C1 and C2. The circuit components are interconnected by wiresto the printed circuit conductors and also to battery output terminals(not shown) extending from the battery case C.

Referring now to FIGS. 5 and 6, there is shown in each a conventionalacoustic transducer VCl mounted inside the guitar body 14. .A dynamicspeaker may be used in lieu of the transducer if desired. In FIG. 5 aplan view is shown of the rear end portion of the guitar body 14,corresponding to that portion to the right of the sound opening 12 inFIG. 1, with the front face of the body taken away. The transducer VC1is shown mounted on one of two internal guitar frame members 70 thattransversely extend inside the body of the guitar. These members 70,together with the front wall 14a, back wall 14b and side wall 140 ofguitar body 14, formed an enclosed space 72 into which the acoustictransducer portion 73 may broadcast. The broadcasting takes place-bymoving air in the confined space to take advantage of the acousticalresonance and acoustical amplification that is inherent to the guitarbody 14 when the guitar strings are plucked or strummed.

Referring now to the circuitry shown in FIG. 7, the

circuitry includes optoelectronic transducer circuitry 76 electricallycoupled to a preamplifier circuit 78 that in turn is electricallycoupled to a power amplifier circuit Q1 which has its output terminalsconnected to the loudspeaker VC]. The circuitry has a direct currentpower source Bl, preferably a battery inside the guitar body 14, thatsupplies direct current thereto when a switch W1, operated by the switchbutton 20 (see FIG. 1), is closed. j

The circuit components shown in FIG. 7 are labelled for identification,and are identified and rated as follows: D1 through D6 are all lightemitters; D7 through D12 are all light sensors; D13 through D18 are 1N67rectifying diodes; R1 through R25 are ohmic resistors; R1 and R2 are0.50 watt 330 ohm resistors; R3, R5, R7, R9, R11, and R13 are 0.25 watt100,000 ohm resistors; R4, R6, R8, R10, R12, and R14 are 0.25 watt1,500,000 ohm resistors; R15 is a 0.25 watt 47,000 ohm resistor; R16 andR17 are 0.25 watt 3,300,000 ohm resistors; R18 is a 0.25 watt 10,000 ohmresistor; R19 is a 500,000 ohm potentiometer; R20 is a 0.25 watt 500,000ohm resistor; R21 is a 0.25 watt 56,000 ohm resistor; R22 is a 0.25 watt18,000 ohm resistor; R23 is a 0.25 watt 330,000 ohm resistor; R24 is a0.25 watt 56,000 ohm resistor; R25 is a 0.25 watt 5,600 ohm resistor; C1through C8 are capacitors rated in micro- :farads as follows: C1 is0.005 microfarads, C2 is 2.2, C3 is 47, C4 is 1,000, C5 is 30, C6 is500, C7 is 4.7 and C8 is 0.002 microfarads. B1 is a battery supplyconsisting of 10 series-connected nickel cadmium battery cells. O1 is aGeneral Electric integrated circuit (PA 237) power amplifier circuit.VCl is either an acoustic transducer or a dynamic speaker with an 8 ohmvoice coil. 02 is an NPN emitter-base-collector transistor.

The light emitters D1, D2, and D3 are connected in series with theresistor R1, and the light emitters D4, D5, and D6 are connected inseries with the resistor R2. The emitters D1, D2 and D3 and the resistorR1 are connected in parallel with the emitters D4, D5, and D6 and theresistor R2. The emitters D1 through D6 are all poled in the samedirection so that each is normally forward biased during circuitoperation so as to each emit a light beam. The light sensors D7 throughD12 are connected in respective series circuits that are parallel to oneanother and also in series with the resistor R15 that is itself inparallel with the capacitor C1.

The resistor R3 and the diode D13 are connected in series with thephotodiode D7. The resistor R5 and the diode D14 are connected in serieswith the photodiode D8. The resistor R7 and the diode D15 are connectedin series with the photodiode D9. The resistor R9 and the diode D16 areconnected in series with the photodiode D10. The resistor R11 and thediode D17 are connected in series with the photodiode D11. The resistorR13 and the diode D18 are connected in series with the photodiode D12.

The light sensors and diodes D7 through D18 are all poled in the samedirection so as to be normally forward biased during circuit operation.The resistors R4, R6, R8, R10, R12, and R14 are respectively connectedin series with the sensors D7 through D12. The sensors D7 through D12are respectively illuminated by the light rays coming from the emittersD1 through D6. Each illuminated sensor has a voltage induced therein bythe light rays impinging thereon.

Sensor D7 and resistor R4, sensor D8 and resistor R6, sensor D9 andresistor R8, sensor D10 and resistor R10, sensor D11 and resistor R12,and sensor D12 and resistor R14, respectively, are connected in parallelwith the series circuit including D1, D2 and D3 and resistor R1; andalso with the series circuit including diodes D4, D5, and D6 andresistor R2. The light emitters D1 through D6, and light sensors D7through D12, resistors R1 through R15 and the capacitor C1 are connectedtogether to form the optoelectronic transducer pickup circuit 76.

The preamplifier circuit 78 includes the bias resistors R16, R17 and R18and the NPN emitter-base-collector transistor Q2. The resistor R18,connected to the emitter of the transistor Q2, serves as an emitterfollower resistor, the resistor R16 being connected in parallel with thebase and collector of the transistor 02 serves to reverse bias thebase-collector transistor junction when the switch W1 is closed. Theresistor R17 is connected in parallel with the base and emitter of thetransistor Q2 and the resistor R18'serves as a base resistor forapplying a signal voltage to the transistor emitterbase junction tomodulate the forward bias thereof when a signal voltage at the cathodesof the diodes D13 through D18 is coupled thereto by the capacitor C2.The capacitor C2 serves as a signal coupling capacitor connected betweenthe output of the optoelectronic transducer pickup circuit 76 and thebase of the transistor Q2 of the preamplifier circuit 78. the capacitorC4 connected in parallel with the battery Bl serves to suppressoscillations that might otherwise occur, but for the capacitor C4 in thepreamplifier circuit.

The capacitor C3 and the potentiometer R19 are series connected inparallel with the resistor R18 so that amplified alternating currentsignal voltages impressed across the resistor R18, resulting from theapplication of signal voltage across the transistor Q2 emitter-basejunction, are coupled by the capacitor C3 to the potentiometer R19. Thepotentiometer R19 has a movable tap terminal for pickup of a selectedportion of any voltage impressed on the resistance of the potentiometerR19. The position of the potentiometer tap terminal is adjustable byturning a volume control knob K (see FIG. 1) extending from the casing18 of the unit 16. The position of this tap terminal determines thevolume of sound that can issue from the loudspeaker VCl.

Resistors R20 and R21 are series connected, and together are connectedin parallel with the resistors R16 and R17. The capacitor C is acoupling capacitor that is connected between the potentiometer tapterminal and the serial junction of the resistor R and resistor R21. Theresistors R20 and R21 serve as a voltage divider in circuit with thebattery B1, and together function to establish a bias voltage level atan input terminal T14 of the power amplifier integrated circuit Q1.

The power amplifier circuit Q1 includes, in addition to the inputterminal T14, output terminals T3 and T5 and circuit biasing terminalsT8, T7 and T12. The coupling resistor R22 is connected in series withthe two loudspeaker terminals between the power amplifier outputterminals T3 and T5 to couple the output of the power amplifier circuitto the loudspeaker VCl.

A biasing circuit network, including the resistors R23, R and R25 andthe capacitors C6, C7 and C8, is interconnected with the power amplifiercircuit Q1 and the acoustic transducer VCl. The resistor R23 isconnected between the terminals T7 and T12; the resistor R24 isconnected between the terminals T12 and T8; and the resistor R25 and thecapacitor C7 are series connected across the resistor R24. The capacitorC8 is connected between the terminals T3 and T7. The capacitor C6 isconnected between one end of the resistor R22 and the terminal T7. Theterminal T5 is connected to the collector of the transistor 02. Voltagesignals applied across the resistor R21 are amplified by the poweramplifier circuit Q2 to provide output signals of amplified power at theterminals T3 and T5 that are coupled to the acoustic transducer VCl bythe resistor R22 to drive the transducer.

The acoustic transducer VC1, when driven, broad casts the amplifiedsound of the vibrating strings to the confined space 72 to be furtheramplified as the guitar body 14 acts as a resonator.

From the above description it will be apparent that the strings S1through S6 must be formed from a material that is less light conductivethan air in order that the vibrating strings will stop the beams oflight prior to the latter impinging on the light sensors D6 to D12.

Light of variable intensity resulting from the vibration of the stringsS4 to S-6 may be used to provide an electrical output that isproportional thereto by the device shown in FIG. 8 that is provided foreach of the strings. Each of these devices Z includes two NPN junctions200 and 202. The NPN junctions 200 and 202 are placed in an envelope 204that is transparent to the extent that a light beam from one of thelight emitters D-l to D-6 impinges on the junctions 200 and 202. The PNjunction formed in silicon, which is the basic structural element of thesemiconductor diode and transistor, is inherently photosensitive. Thatis to say that such a junction, when electrically stressed in thereverse polarity passes a current which is dependent upon secondarysources of energy such as heat or light. The greater the amount of lightenergy irradiating such a junction the greater the current passingthrough the junction. All PN junctions exhibit this phenomena in varyingdegrees, and the processes required to enhance this characteristic areclearly understood throughout the semiconductor industry.

The base 200a, collector 200b, and emitter 200c of NPN junction 200 areconnected by conductors 206, 208 and 210 to a resistor 212, junctionpoint 214 and base 202a of NPN junction 202. Collector 202b is connectedto junction point 214. Emitter 2020 is connected by a conductor 216 toresistor 218. Resistors 212 and 218 are connected to a conductor 220that has a junction point 220a therein that is connected by a conductor222 to ground 224.

Junction point 214 is connected by a conductor 226 to a junction point228, which junction point is connected to a resistor 230. Resistor 230is connected to a conductor 232 that extends to a junction point 234.Connector 220 terminates in a junction point 236. Power is supplied tojunction points 234 and 236. The output voltage that is determined bythe intensity of the light beam is taken off from conductor 220 and aconductor 240 connected to junction point 228.

Recognizing the fact that any transistor is made up of two PN junctionsarranged in such a manner that they have an electrically common area(i.e., the base), and that under normal operating conditions one ofthese junctions is reverse polarized (e.g., base-tocollector) while theother is forward polarized (e.g., base-to-emitter), it may be recognizedthat the photo current is generated within the base-to-collectorjunction and is passed through the base-to-emitter junction in serieswith the load circuit (i.e., collector-to-supplyto-emitter).Consequently, it is feasible for the photo current to forward polarizethe base-to-emitter junction without the use of external electricalattachment.

In order to continue this description, it must first be clarified thatforward polarization of the base-toemitter junction is normal operation,but as is the case with the base to collector junction, thebase-toemitter junction may continue to pass current even when reversepolarized, due to ambient thermal energy. Also, it should be noted thatphoto current must pass through the base-to-emitter junction in order tobe amplified by normal transistor action.

Continuing, by connecting a resistor in the load circuit between theemitter and the reference (i.e., ground) end of the supply, a potentialis developed across this resistor that is proportional to thephotocurrent and is in phase with respect to changes in illuminationwhen measured with respect to the reference. By connecting a secondresistor between the base electrical attachment and the reference (i.e.,in parallel with the base-to-emitter junction and emitter to referenceresistance) a portion of the photocurrent is diverted around thebase-to-emitter junction. This current gives rise to a potential of thesame polarity and phase as the potential across the emitter-to-referenceresistor. The difference between these two potentials is thebase-to-emitter junction polarization potential, and will beself-adjusting. That is to day that the photo current will dividebetween the two available paths in such proportions as to establishquiescent equilibrium under all ambient conditions.

It should be clear at this point that connection of the resistancebetween the base electrical attachment and the reference has given riseto a biasing current that is directly proportional but 180 out of phasewith photocurrent (i.e., illumination). This technique of negativecurrent feed-back has all of the advantageous effects of negativefeed-back as used in more conventional circuitry. Specific areas ofimprovement include (1) gain stabilization device-to-device as well aswith changes in ambient conditions, (2) extended bandwidth, (3) reduceddistortion, and (4) improved isolation of the collector-to-battery(i.e., output) circuit.

I claim:

1. In combination with a musical instrument having a sound box in whichan opening is formed over which a plurality of parallel, laterallyspaced, tensioned strings extend that are formed from a material that isless light conductive than air, an assembly operatively associated withsaid musical instrument for amplifying the sound from said strings whenthe latter are vibrated, said assembly including:

a. electrically operated, sound reproduction means;

b. a source of electric power;

0. electric amplifier circuit means that connects said soundreproduction means and said source of electric power; and

d. a plurality of electrically operated light emitting and light sensingmeans located on opposite sides of said strings and connected to saidelectric amplifier circuit means, with said strings so situated as to atleast partially obstruct the beams of light between said light emittingand light sensing means, said strings when vibrating cooperating withsaid light emitting means to subject said light sensing means to aplurality of spaced pulses of light of the same frequencies as those atwhich said strings vibrate, and said light sensing means impartingelectric signals to said electric amplifier circuit means to cause saidsound reproduction means to reproduce the sounds of said vibratingsprings at an amplified level, with said light sensing means includl.first and second NPN junctions that each includes a base, collector andemitter; 2. first, second and third resistors;

3. a first plurality of conductors that connect the base of said firstNPN junction to said first resistor, the emitter of said second NPNjunction to said second resistor, the collector of said second emitterto said third resistor, the collector of said first NPN junction to thecollector of said second PN junction, and the emitter of said first NPNjunction to the base of said second NPN junction;

4. a second conduit that connects said first and second resistors to theground;

5. a third conductor connected to said third resistor, with said second.and third conductors connected to said source of power; and

6. a fourth conductor connected to said collector of said second NPNjunction, and said signal being emitted by said third and fourthconductors.

2. An assembly as defined in claim 1 in which said instrument is anacoustic guitar, and with said sound reproduction means so positionedwithin said sound box as to direct the reproduced sounds of said stringsaway from said opening, and said sound box acting as a resonator tofurther amplify the reproduced amplified sounds of said strings fromsaid sound reproduction means.

3. An assembly as defined in claim 2 in which said sound reproductionmeans is an acoustic transducer.

4. An assembly as defined in claim 2 in which said sound reproductionmeans is a dynamic speaker.

5. An assembly as defined in claim 1 which in addition includes:

e. means for effecting relative lateral adjustment between said stringsand said light emitting and light sensing means whereby said stringsobstruct at least portions of said beams of light prior to the latterimpinging on said light sensing means.

6. An assembly as defined in claim 1 in which said musical instrumentincludes a bridge over which said tensioned strings extend, and saidassembly in addition including:

e. a plurality of laterally adjustable supports mounted on said soundbox adjacent said bridge, with each of said supports having one of saidlight emitting means and light sensing means mounted in spacedrelationship thereon, and each of said supports being movable relativeto one of said strings to a position where said string obstructs atleast a portion of the beam of light from said light emitting means whensaid string is stationary.

' 7. An assembly as defined in claim 1 in which said source of electricpower is at least one battery removably situated at a fixed positionrelative to said sound box.

1. In combination with a musical instrument having a sound box in whichan opening is fOrmed over which a plurality of parallel, laterallyspaced, tensioned strings extend that are formed from a material that isless light conductive than air, an assembly operatively associated withsaid musical instrument for amplifying the sound from said strings whenthe latter are vibrated, said assembly including: a. electricallyoperated, sound reproduction means; b. a source of electric power; c.electric amplifier circuit means that connects said sound reproductionmeans and said source of electric power; and d. a plurality ofelectrically operated light emitting and light sensing means located onopposite sides of said strings and connected to said electric amplifiercircuit means, with said strings so situated as to at least partiallyobstruct the beams of light between said light emitting and lightsensing means, said strings when vibrating cooperating with said lightemitting means to subject said light sensing means to a plurality ofspaced pulses of light of the same frequencies as those at which saidstrings vibrate, and said light sensing means imparting electric signalsto said electric amplifier circuit means to cause said soundreproduction means to reproduce the sounds of said vibrating springs atan amplified level, with said light sensing means including:
 1. firstand second NPN junctions that each includes a base, collector andemitter;
 2. first, second and third resistors;
 3. a first plurality ofconductors that connect the base of said first NPN junction to saidfirst resistor, the emitter of said second NPN junction to said secondresistor, the collector of said second emitter to said third resistor,the collector of said first NPN junction to the collector of said secondPN junction, and the emitter of said first NPN junction to the base ofsaid second NPN junction;
 4. a second conduit that connects said firstand second resistors to the ground;
 5. a third conductor connected tosaid third resistor, with said second and third conductors connected tosaid source of power; and
 6. a fourth conductor connected to saidcollector of said second NPN junction, and said signal being emitted bysaid third and fourth conductors.
 2. first, second and third resistors;2. An assembly as defined in claim 1 in which said instrument is anacoustic guitar, and with said sound reproduction means so positionedwithin said sound box as to direct the reproduced sounds of said stringsaway from said opening, and said sound box acting as a resonator tofurther amplify the reproduced amplified sounds of said strings fromsaid sound reproduction means.
 3. An assembly as defined in claim 2 inwhich said sound reproduction means is an acoustic transducer.
 3. afirst plurality of conductors that connect the base of said first NPNjunction to said first resistor, the emitter of said second NPN junctionto said second resistor, the collector of said second emitter to saidthird resistor, the collector of said first NPN junction to thecollector of said second PN junction, and the emitter of said first NPNjunction to the base of said second NPN junction;
 4. a second conduitthat connects said first and second resistors to the ground;
 4. Anassembly as defined in claim 2 in which said sound reproduction means isa dynamic speaker.
 5. An assembly as defined in claim 1 which inaddition includes: e. means for effecting relative lateral adjustmentbetween said strings and said light emitting and light sensing meanswhereby said strings obstruct at least portions of said beams of lightprior to the latter impinging on said light sensing means.
 5. a thirdconductor connected to said third resistor, with said second and thirdconductors connected to said source of power; and
 6. a fourth conductorconnected to said collector of said second NPN junction, and said signalbeing emitted by said third and fourth conductors.
 6. An assembly asdefined in claim 1 in which said musical instrument includes a bridgeover which said tensioned strings extend, and said assembly in additionincluding: e. a plurality of laterally adjustable supports mounted onsaid sound box adjacent said bridge, with each of said supports havingone of said light emitting means and light sensing means mounted inspaced relationship thereon, and each of said supports being movablerelative to one of said strings to a position where said stringobstructs at least a portion of the beam of light from said lightemitting means when said string is stationary.
 7. An assembly as definedin claim 1 in which said source of electric power is at least onebattery removably situated at a fixed position relative to said soundbox.